Aff Plus instance does not meet the right identy law?

We have:
```
instance altAff :: Alt Aff where
  alt a1 a2 = catchError a1 (const a2)

instance plusAff :: Plus Aff where
  empty = throwError (error "Always fails")
```
and Plus right identity law:
```
x `alt` empty == x
```
for `x = throwError (error "whatever")` the above does not hold or am I missing something? Or is it intentional "up to interpretation" meeting the law? 
